A former military chief of Cambodia's notorious Khmer Rouge "killing fields" regime and alleged perpetrator of many of its worst atrocities, Ta Mok, died today in hospital in the capital, Phnom Penh.

The death of the one-legged fighter known as "the Butcher", who replaced the ultra-Maoist movement's leader, Pol Pot, in a 1997 putsch, is a major blow to the international tribunal that started work this month to try the regime's surviving leaders.

Ta Mok, born Ung Choeun, who was arrested on genocide and crimes against humanity charges in 1999, was one of only two Khmer Rouge cadres in detention awaiting trial. His lawyer said he fell into a coma last week after suffering from high blood pressure, tuberculosis and respiratory complications.

Military doctor Tuoth Nara said Ta Mok, who is thought to have been about 82, "died of natural causes, given his poor health and respiratory problems".

Youk Chhang, the head of the Documentation Centre of Cambodia, which has spent decades gathering testimony and evidence about the 1975-1979 regime in which 1.7 million Cambodians - a quarter of the population - were either killed or worked and starved to death, says that both the Cambodian government and United Nations should take some responsibility for his death.

"The government has denied access or adequate services to take care of him while the tribunal did not make any effort to engage with him for the sake of the victims," he told the Guardian.

"After being in jail for so long and then hearing a tribunal is about to start is bound to affect any criminal, cause psychological distress. But the UN stayed silent. Their job is to break the cycle of suspicion so we can all move forward. But they didn't. Are they here for themselves or the victims?"

Reach Sambath, a tribunal spokesman, said he did not want to comment on individuals. "The aim is to try the whole regime," he said. "We're focusing on the most senior leaders and most responsible people."

He acknowledged that Ta Mok was a senior commander, "but there are still many other sources we can seek information from".

Ta Mok's family said his body will be taken for traditional Buddhist rites to Anlong Veng, in mountainous north-western Cambodia where many Khmer Rouge fighters fled after the Vietnamese overthrew the regime in 1979.

They declined to comment, but his lawyer issued a statement last week in which Ta Mok was quoted as saying: "Please inform the whole world that I have never killed anyone."

Ta Mok claims he was only responsible for building bridges, dams and roads, and overseeing rice planting and poultry farming. There is a consensus, however, he was in charge of many of the most brutal crackdowns, dispatched troops to areas which had not yet adopted the regime's "year zero" philosophy and initiated some internal purges in later years.

The only Khmer Rouge commander now in detention is Kaing Khek Lev, known as Duch, who ran the notorious Toul Sleng, or S-21, prison in Phnom Penh.